Microwave anneal of a thin lamina for use in a photovoltaic cell

ABSTRACT

A cleave plane is defined in a semiconductor donor body by implanting ions into the wafer. A lamina is cleaved from the donor body, and a photovoltaic cell is formed which comprises the lamina. The implant may cause some damage to the crystal structure of the lamina. This damage can be repaired by annealing the lamina using microwave energy. If the lamina is bonded to a receiver element, the receiver element may be either transparent to microwaves, or may reflect microwaves, while the semiconductor material absorbs the microwaves. In this way the lamina can be annealed at high temperature while the receiver element remains cooler.

BACKGROUND OF THE INVENTION

The invention relates to a method to use microwave energy to anneal athin semiconductor lamina for use in a photovoltaic cell.

Crystalline damage in semiconductor material such as silicon can berepaired by various means. One of the simplest ways is to subject thesilicon body to a high-temperature anneal. In some circumstances,however, a thermal anneal may present difficulties, as when othermaterials are present that cannot tolerate the anneal temperature.

SUMMARY OF THE PREFERRED EMBODIMENTS

The present invention is defined by the following claims, and nothing inthis section should be taken as a limitation on those claims. Ingeneral, the invention is directed to a method to anneal a semiconductorlamina using microwave energy.

A first aspect of the invention provides for a method to form aphotovoltaic cell, the method comprising the steps of: providing asemiconductor lamina bonded to a receiver element, the lamina having afirst surface and a second surface opposite the first, the thicknessbetween the first surface and the second surface between about 1.5microns and about 10 microns, wherein the lamina is bonded to thereceiver element at the first surface, with zero, one, or more layersintervening, and the second surface is exposed, and wherein the receiverelement has a thickness of at least 80microns; and annealing the entirethickness of the bonded lamina with microwave energy, wherein the laminais suitable for use in the photovoltaic cell, and wherein thephotovoltaic cell comprises the lamina.

Another aspect of the invention provides for a method to form aphotovoltaic cell, the method comprising the steps of: defining a cleaveplane in a substantially crystalline semiconductor donor wafer; bondingthe donor wafer at a first surface to a receiver element; cleaving asubstantially crystalline semiconductor lamina from the donor wafer atthe cleave plane, wherein the lamina remains bonded to the receiverelement at the first surface and a second surface of the lamina iscreated by cleaving, the second surface opposite the first, and whereinthe lamina has a thickness of at least 2 microns; and annealing theentire thickness of the bonded lamina with microwave energy to repaircrystalline defects, wherein the lamina is suitable for use in thephotovoltaic cell, the photovoltaic cell comprising the lamina.

Each of the aspects and embodiments of the invention described hereincan be used alone or in combination with one another.

The preferred aspects and embodiments will now be described withreference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a prior art photovoltaic cell.

FIGS. 2 a-2 d are cross-sectional views of stages of fabrication of aphotovoltaic cell formed according to an embodiment of U.S. patentapplication Ser. No. 12/026530.

FIGS. 3 a-3 f are cross-sectional views showing stages of fabrication ofa photovoltaic cell formed according to an embodiment of the presentinvention.

FIG. 4 is a cross-sectional view showing another embodiment of thepresent invention.

FIG. 5 is a cross-sectional view showing another embodiment of thepresent invention.

FIG. 6 is a cross-sectional view showing yet another embodiment of thepresent invention.

FIG. 7 is a cross-sectional view showing still another embodiment of thepresent invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A conventional prior art photovoltaic cell includes a p-n diode; anexample is shown in FIG. 1. A depletion zone forms at the p-n junction,creating an electric field. Incident photons (incident light isindicated by arrows) will knock electrons from the valence band to theconduction band, creating free electron-hole pairs. Within the electricfield at the p-n junction, electrons tend to migrate toward the n regionof the diode, while holes migrate toward the p region, resulting incurrent, called photocurrent. Typically the dopant concentration of oneregion will be higher than that of the other, so the junction is eithera p+/n− junction (as shown in FIG. 1) or a n+/p− junction. The morelightly doped region is known as the base of the photovoltaic cell,while the more heavily doped region, of opposite conductivity type, isknown as the emitter. Most carriers are generated within the base, andit is typically the thickest portion of the cell. The base and emittertogether form the active region of the cell. The cell also frequentlyincludes a heavily doped contact region in electrical contact with thebase, and of the same conductivity type, to improve current flow. In theexample shown in FIG. 1, the heavily doped contact region is n-type.

Sivaram et al., U.S. patent application Ser. No. 12/026530, “Method toForm a Photovoltaic Cell Comprising a Thin Lamina,” filed Feb. 5, 2008,owned by the assignee of the present invention and hereby incorporatedby reference, describes fabrication of a photovoltaic cell comprising athin semiconductor lamina formed of non-deposited semiconductormaterial. Referring to FIG. 2 a, in embodiments of Sivaram et al., asemiconductor donor wafer 20 is implanted through first surface 10 withone or more species of gas ions, for example hydrogen and/or heliumions. The implanted ions define a cleave plane 30 within thesemiconductor donor wafer. As shown in FIG. 2 b, donor wafer 20 isaffixed at first surface 10 to receiver 60. Referring to FIG. 2 c, ananneal causes lamina 40 to cleave from donor wafer 20 at cleave plane30, creating second surface 62. In embodiments of Sivaram et al.,additional processing before and after the cleaving step forms aphotovoltaic cell comprising semiconductor lamina 40, which is betweenabout 0.2 and about 100 microns thick, for example between about 0.2 andabout 50 microns, for example between about 1 and about 20 micronsthick, in some embodiments between about 1 and about 10 microns thick,though any thickness within the named range is possible. FIG. 2 d showsthe structure inverted, with receiver 60 at the bottom, as duringoperation in some embodiments. Receiver 60 may be a discrete receiverelement having a maximum width no more than 50 percent greater than thatof donor wafer 10, and preferably about the same width, as described inHerner, U.S. patent application Ser. No. 12/057265, “Method to Form aPhotovoltaic Cell Comprising a Thin Lamina Bonded to a Discrete ReceiverElement,” filed on Mar. 27, 2008, owned by the assignee of the presentapplication and hereby incorporated by reference. Alternatively, aplurality of donor wafers may be affixed to a single, larger receiver,and a lamina cleaved from each donor wafer.

Using the methods of Sivaram et al., photovoltaic cells, rather thanbeing formed from sliced wafers, are formed of thin semiconductorlaminae without wasting silicon through excessive kerf loss or byfabrication of an unnecessarily thick cell, thus reducing cost. The samedonor wafer can be reused to form multiple laminae, further reducingcost, and may be resold after exfoliation of multiple laminae for someother use.

The implant step, during which ions are implanted through the thicknessof semiconductor material which will ultimately become the lamina, maycause damage to the crystal structure of the lamina. This damage reducesthe efficiency of the cell. Localized damage at the new lamina surfacecreated by exfoliation can be removed by various surface treatments, forexample by etching. Removing damage that extends through the entirethickness of the lamina, however, is more problematic.

The present invention provides a method to selectively anneal asemiconductor body without heating adjacent materials to the same hightemperature. Thus, materials that cannot tolerate a high annealtemperature are protected from heat damage. In the present invention,damage to the crystal structure of the lamina is repaired by annealingthe lamina using microwave radiation. Microwave energy is absorbed bythe semiconductor material of the lamina, generating heat within thebody of the lamina and annealing it. The receiver element, if it is madeof a different material, will respond differently to microwave energy.The receiver element may be of various materials, including, forexample, glass, ceramic, metallurgical-grade silicon, or metal. Glassand most ceramics are transparent to microwaves; thus microwave energywill pass through a glass or ceramic receiver element without directlyheating it. In some embodiments, a metal layer or stack, which may serveto reflect light in the completed device, is formed between the laminaand the receiver element. This metal layer may reflect microwaves backthrough the lamina, increasing absorption of microwave energy, andshielding the receiver element from some or all microwave energy. Ametal receiver element may also reflect rather than absorb microwaves.

During a traditional thermal anneal in a furnace, the lamina and thereceiver element are heated to essentially the same temperature. Incontrast, microwave energy is absorbed by the lamina, heating it to atemperature sufficient to cure crystal defects, while microwaves passthrough, or are reflected from, the receiver element; thus the receiverelement may be substantially cooler. A furnace anneal to repair crystaldamage may be performed at temperatures from 600 to 1000 degrees C.Annealing at temperatures exceeding, for example, about 900 degrees C.can be most effective. Not all materials can readily toleratetemperatures this high, however. By lowering the temperature experiencedby the receiver element, the range of materials that can be used for thereceiver element is increased. Some less temperature-tolerant materialsmay be advantageous for a variety of reasons; for example they may beless expensive. In some embodiments, a microwave anneal may be combinedwith a heating step, allowing the heating step to be performed at alower temperature, for a shorter time, or both, than if it wereperformed without the application of microwave energy.

To summarize, what is described is a method to form a photovoltaic cell,the method comprising the steps of: providing a semiconductor laminabonded to a receiver element, the lamina having a first surface and asecond surface opposite the first, the thickness between the firstsurface and the second surface between about 1.5 microns and about 10microns, wherein the lamina is bonded to the receiver element at thefirst surface, with zero, one, or more layers intervening, and thesecond surface is exposed, and wherein the receiver element has athickness of at least 80 microns; and annealing the entire thickness ofthe bonded lamina with microwave energy, wherein the lamina is suitablefor use in the photovoltaic cell, and wherein the photovoltaic cellcomprises the lamina.

For clarity, a detailed example of a photovoltaic assembly including areceiver element and a lamina having thickness between 0.2 and 100microns, in which the lamina is annealed with microwave energy, will beprovided. For completeness, many materials, conditions, and steps willbe described. It will be understood, however, that many of these detailscan be modified, augmented, or omitted while the results fall within thescope of the invention.

EXAMPLE Anneal To Cure Crystal Defects

The process begins with a donor body of an appropriate semiconductormaterial. An appropriate donor body may be a monocrystalline siliconwafer of any practical thickness, for example from about 200 to about1000 microns thick. Typically the wafer has a <100> orientation, thoughwafers of other orientations may be used. In alternative embodiments,the donor wafer may be thicker; maximum thickness is limited only bypracticalities of wafer handling. Alternatively, polycrystalline ormulticrystalline silicon may be used, as may microcrystalline silicon,or wafers or ingots of other semiconductor materials, includinggermanium, silicon germanium, or III-V or II-VI semiconductor compoundssuch as GaAs, InP, etc. In this context the term multicrystallinetypically refers to semiconductor material having grains that are on theorder of a millimeter or larger in size, while polycrystallinesemiconductor material has smaller grains, on the order of a thousandangstroms. The grains of microcrystalline semiconductor material arevery small, for example 100 angstroms or so. Microcrystalline silicon,for example, may be fully crystalline or may include these microcrystalsin an amorphous matrix. Multicrystalline or polycrystallinesemiconductors are understood to be completely or substantiallycrystalline. It will be appreciated by those skilled in the art that theterm “monocrystalline silicon” as it is customarily used will notexclude silicon with occasional flaws or impurities such asconductivity-enhancing dopants.

The process of forming monocrystalline silicon generally results incircular wafers, but the donor body can have other shapes as well. Forphotovoltaic applications, cylindrical monocrystalline ingots are oftenmachined to an octagonal cross section prior to cutting wafers. Wafersmay also be other shapes, such as square. Square wafers have theadvantage that, unlike circular or hexagonal wafers, they can be alignededge-to-edge on a photovoltaic module with minimal unused gaps betweenthem. The diameter or width of the wafer may be any standard or customsize. For simplicity this discussion will describe the use of amonocrystalline silicon wafer as the semiconductor donor body, but itwill be understood that donor bodies of other types and materials can beused.

Referring to FIG. 3 a, donor wafer 20 is a monocrystalline silicon waferwhich is lightly to moderately doped to a first conductivity type. Thepresent example will describe a relatively lightly n-doped wafer 20 butit will be understood that in this and other embodiments the dopanttypes can be reversed. Wafer 20 may be doped to a concentration ofbetween about 1×10¹⁵ and about 1×10¹⁸ dopant atoms/cm³, for exampleabout 1×10¹⁷ dopant atoms/cm³. Donor wafer 20 may be, for example,solar- or semiconductor-grade silicon.

First surface 10 of donor wafer 20 may be substantially planar, or mayhave some preexisting texture. If desired, some texturing or rougheningof first surface 10 may be performed, for example by wet chemical orplasma treatment. Surface roughness may be random or may be periodic, asdescribed in “Niggeman et al., “Trapping Light in Organic Plastic SolarCells with Integrated Diffraction Gratings,” Proceedings of the 17^(th)European Photovoltaic Solar Energy Conference, Munich, Germany, 2001.Methods to create surface roughness are described in further detail inPetti, U.S. patent application Ser. No. 12/130,241, “Asymmetric SurfaceTexturing For Use in a Photovoltaic Cell and Method of Making,” filedMay 30, 2008; and in Herner, U.S. patent application Ser. No.12/343,420, “Method to Texture a Lamina Surface Within a PhotovoltaicCell,” filed Dec. 23, 2008, both owned by the assignee of the presentapplication and both hereby incorporated by reference.

First surface 10 may be heavily doped to some depth to the sameconductivity type as wafer 20, forming heavily doped region 14; in thisexample, heavily doped region 14 is n-type. As wafer 20 has not yet beenaffixed to a receiver element, high temperatures can readily betolerated at this stage of fabrication, and this doping step can beperformed by any conventional method, including diffusion doping. Anyconventional n-type dopant may be used, such as phosphorus or arsenic.Dopant concentration may be as desired, for example at least 1×10¹⁸dopant atoms/cm³, for example between about 1×10¹⁸ and 1×10²¹ dopantatoms/cm³. Doping and texturing can be performed in any order, but sincemost texturing methods remove some thickness of silicon, it may bepreferred to form heavily doped n-type region 14 following texturing.Heavily doped region 14 will provide electrical contact to the baseregion in the completed device.

Next, in the present embodiment, a dielectric layer 28 is formed onfirst surface 10. As will be seen, in the present example first surface10 will be the back of the completed photovoltaic cell, and a conductivematerial is to be formed on the dielectric layer. The reflectivity ofthe conductive layer to be formed is enhanced if dielectric layer 28 isrelatively thick. For example, if dielectric layer 28 is silicondioxide, it may be between about 1000 and about 1500 angstroms thick,while if dielectric layer 28 is silicon nitride, it may be between about700 and about 800 angstroms thick, for example about 750 angstroms. Thislayer may be grown or deposited by any suitable method. A grown oxide ornitride layer 28 passivates first surface 10 better than if this layeris deposited. In some embodiments, a first thickness of dielectric layer28 may be grown, while the rest is deposited.

In the next step, ions, preferably hydrogen or a combination of hydrogenand helium, are implanted into wafer 20 to define cleave plane 30, asdescribed earlier. This implant may be performed using the implanterdescribed in Parrill et al., U.S. patent application Ser. No. 12/122108,“Ion Implanter for Photovoltaic Cell Fabrication,” filed May 16, 2008;or those of Ryding et al., U.S. patent application Ser. No. 12/494,268,“Ion Implantation Apparatus and a Method for Fluid Cooling,” filed Jun.30, 2009; or of Purser et al. U.S. patent application Ser. No.12/621,689, “Method and Apparatus for Modifying a Ribbon-Shaped IonBeam,” filed Nov. 19, 2009, all owned by the assignee of the presentinvention and hereby incorporated by reference. The overall depth ofcleave plane 30 is determined by several factors, including implantenergy. The depth of cleave plane 30 can be between about 0.2 and about100 microns from first surface 10, for example between about 0.5 andabout 20 or about 50 microns, for example between about 1 and about 10microns or between about 1 or 2 microns and about 5 or 6 microns.

Turning to FIG. 3 b, after implant, openings 33 are formed in dielectriclayer 28 by any appropriate method, for example by laser scribing orscreen printed etchant paste. The size of openings 33 may be as desired,and will vary with dopant concentration, metal used for contacts, etc.In one embodiment, these openings may be about 40 microns square. Notethat figures are not to scale.

A titanium layer 24 is formed on dielectric layer 28 by any suitablemethod, for example by sputtering or thermal evaporation. This layer mayhave any desired thickness, for example between about 20 and about 2000angstroms, in some embodiments about 300 angstroms thick or less, forexample about 100 angstroms. Layer 24 may be titanium or an alloythereof, for example, an alloy which is at least 90 atomic percenttitanium. Titanium layer 24 is in immediate contact with first surface10 of donor wafer 20 in vias 33; elsewhere it contacts dielectric layer28. In alternative embodiments, dielectric layer 28 is omitted, andtitanium layer 24 is formed in immediate contact with donor wafer 20 atall points of first surface 10.

Non-reactive barrier layer 26 is formed on and in immediate contact withtitanium layer 24. This layer is formed by any suitable method, forexample by sputtering or thermal evaporation. Non-reactive barrier layer26 may be any material, or stack of materials, that will not react withsilicon, is conductive, and will provide an effective barrier to thelow-resistance layer to be formed in a later step. Suitable materialsfor non-reactive barrier layer include TiW, TiN, W, Ta, TaN, TaSiN, Ni,Mo, Zr, or alloys thereof. The thickness of non-reactive barrier layer26 may range from, for example, between about 100 and about 10,000angstroms. In some embodiments this layer is about 1000 angstroms thick.

Low-resistance layer 22 is formed on non-reactive barrier layer 26. Thislayer may be, for example, silver, cobalt, or tungsten or alloysthereof. In this example low-resistance layer 22 is silver or an alloythat is at least 90 atomic percent silver, formed by any suitablemethod. Silver layer 22 may be between about 5000 and about 100,000angstroms thick, for example about 20,000 angstroms (2 microns) thick.

In this example an adhesion layer 32 is formed on low-resistance layer22. Adhesion layer 32 is a material that will adhere to receiver element60, for example titanium or an alloy of titanium, for example an alloywhich is at least 90 atomic percent titanium. In alternativeembodiments, adhesion layer 32 can be a suitable dielectric material,such as Kapton or some other polyimide. In some embodiments, adhesionlayer 32 is between about 100 and about 5000 angstroms, for exampleabout 400 angstroms.

In alternative embodiments, some of the layers making up intermetalstack 21, such as adhesion layer 32 and low-resistance layer 22, couldbe deposited on receiver element 60 instead of on donor wafer 20.

Next, wafer 20 is affixed to a receiver element 60, with dielectriclayer 28, titanium layer 24, non-reactive barrier layer 26,low-resistance layer 22, and adhesion layer 32 intervening. Receiverelement 60 may be any suitable material, including glass, such assoda-lime glass or borosilicate glass; a metal or metal alloy such asstainless steel or aluminum; a polymer such as a polyimide; a ceramic;or a semiconductor, such as metallurgical grade silicon. Receiverelement 60 may be a laminate structure, including layers of differentmaterials. In general, receiver element 60 will be at least 80 micronsthick, for example 200 microns thick or more. The wafer 20, receiverelement 60, and intervening layers are bonded by any suitable method,for example by anodic bonding. In some embodiments, receiver element 60has a widest dimension no more than about twenty percent greater thanthe widest dimension of wafer 20, and in most embodiments the widestdimension may be about the same as that of wafer 20. In otherembodiments, receiver element 60 is significantly larger than wafer 20,and additional donor wafers may be bonded to the same receiver element.

Referring to FIG. 3 c, which shows the structure inverted with receiverelement 60 on the bottom, a thermal step causes lamina 40 to cleave fromthe donor wafer at the cleave plane. In some embodiments, this cleavingstep may be combined with a bonding step. Cleaving is achieved in thisexample by exfoliation, which may be achieved at temperatures between,for example, about 350 and about 650 degrees C. In general, exfoliationproceeds more rapidly at higher temperature. In some embodiments,exfoliation may be achieved using a microwave anneal, or a microwavetreatment combined with a furnace anneal. The thickness of lamina 40 isdetermined by the depth of cleave plane 30. In many embodiments, thethickness of lamina 40 is between about 1and about 10 microns, forexample between about 2 and about 5 microns, for example about 4.5microns. Bonding and exfoliation may be achieved using methods describedin Agarwal et al., U.S. patent application Ser. No. 12/335,479, “Methodsof Transferring a Lamina to a Receiver Element,” filed Dec. 15, 2008,owned by the assignee of the present application and hereby incorporatedby reference.

During relatively high-temperature steps, such as the exfoliation oflamina 40, the portions of titanium layer 24 in immediate contact withsilicon lamina 40 will react to form titanium silicide. If dielectriclayer 28 was included, titanium silicide is formed where first surface10 of lamina 40 was exposed in vias 33. If dielectric layer 28 wasomitted, in general all of the titanium of titanium layer 24 will beconsumed, forming a blanket of titanium silicide.

Second surface 62 has been created by exfoliation. Second surface 62will typically have some damage, and steps may be taken to remove orrepair this damage. Some damage may be removed by wet etching, forexample with KOH or TMAH. Some thickness of silicon will be removed bythis etch, for example between about 3000 to 7000 angstroms or more. Ingeneral a deeper implant (resulting in a thicker lamina) will have athicker damaged zone to be removed. An etch step intended to create sometexture at this surface to increase internal reflection may be combinedwith the damage-removal etch, or may be performed independently. Oneoption, for example, is the self-limiting etch described by Clark etal., U.S. patent application Ser. No. 12/484271, “Selective Etch ForDamage Removal at Exfoliated Surface,” filed Jun. 15, 2009, owned by theassignee of the present application and hereby incorporated byreference.

At this point lamina 40 may be exposed to a microwave anneal to repairpoint defects within the body of the lamina. Microwave energy may havewavelength ranging from 1 mm to 1 m. The frequency may be, for example,2.45 GHz. The power of the microwave may be, for example, between about100 watts and about 6000 watts, for example between about 500 and about5000 watts, in some embodiments between about 1000 and about 1500 watts.

Turning to FIG. 3 d, in one embodiment microwaves may be applied from amicrowave source from an angle, as in the figure, or from directlyabove. The lamina-receiver element assembly may be rotated or otherwisemoved during microwave treatment or may be stationary; likewise, themicrowave source may be scanning If receiver element 60, or some layerin the stack between receiver element 60 and lamina 40, is formed ofmetal, or some other reflective material, that layer may reflectmicrowaves. In this case, some incident microwaves 50 will be absorbedby the lamina 40, and the microwaves that reach the reflective layerwill be reflected back into lamina 40. In the example just described,microwaves will be reflected from titanium layer 24, titanium nitridelayer 26, and/or silver layer 22, as shown in FIG. 3 d. If the metallayer or stack is relatively thick, little or no microwave energy isexpected to reach receiver element 60. A thinner layer or stack, perhapsthousands of angstroms, will reflect some but not all microwave energy,reducing the amount that reaches receiver element 60.

The microwaves may be applied for any suitable amount of time, forexample from a few seconds through thirty to forty minutes, for examplefrom two to five minutes. The microwaves may be continuous or pulsed.The microwave anneal may be combined with a thermal anneal. For example,microwave annealing may be performed while the lamina 40 and receiverelement 60, and associated layers, are heated to about 200 degrees C. Inother embodiments, microwave annealing may be performed without heatingthe lamina and receiver element assembly. Microwave energy enters lamina40 at second surface 62, and penetrates its entire thickness. The laminamay be, for example 1 to 20 microns thick, for example 1 to 10 micronsthick, for example 1 to 5 microns thick, for example 4 microns thick. Inmost embodiments, then, microwave energy, is absorbed at a depth of atleast 2, or at least 4 microns beneath second surface 62. In someembodiments, all or a portion of the microwave anneal may be performedin an ambient of hydrogen or forming gas. The hydrogen may serve topassivate interstitial defects.

Referring to FIG. 3 e, if any native oxide (not shown) has formed onsecond surface 62, it may be removed by any conventional cleaning step,for example by etching in dilute hydrofluoric acid. The surface canoptionally be passivated by microwave decomposition of ozone at secondsurface 62 to form a thin oxide (not shown), for example about 15 toabout 30 angstroms thick. This layer is thin enough to allow tunnelingof charge carriers, and need not be removed.

Next, a silicon layer is deposited on second surface 62, directly on thepassivating oxide, if formed. This layer 74 includes heavily dopedsilicon, and may be amorphous, microcrystalline, nanocrystalline, orpolycrystalline silicon, or a stack including any combination of these.This layer or stack may have a thickness, for example, between about 100and about 350 angstroms. FIG. 3 e shows an embodiment that includesintrinsic amorphous silicon layer 72 between second surface 62 and dopedlayer 74. In other embodiments, layer 72 may be omitted. In thisexample, heavily doped silicon layer 74 is doped p-type, opposite theconductivity type of lightly doped n-type lamina 40, and serves as theemitter of the photovoltaic cell being formed, while lightly dopedn-type lamina 40 comprises the base region. If included, layer 72 issufficiently thin that it does not impede electrical connection betweenlamina 40 and doped silicon layer 74.

A transparent conductive oxide (TCO) layer 110 is formed on heavilydoped silicon layer 74. Appropriate materials for TCO 110 include indiumtin oxide, as well as aluminum-doped zinc oxide, tin oxide, titaniumoxide, etc.; this layer may be, for example, about 1000 angstroms thick,and serves as both a top electrode and an antireflective layer. Inalternative embodiments, an additional antireflective layer (not shown)may be formed on top of TCO 110.

A photovoltaic cell has been formed, including lightly doped n-typelamina 40, which comprises the base of the cell, and heavily dopedp-type amorphous silicon layer 74, which serves as the emitter of thecell. Heavily doped n-type region 14 will improve electrical contact tothe cell. Electrical contact must be made to both faces of the cell.This contact can be formed using a variety of methods, including thosedescribed in Petti et al., U.S. patent application Ser. No. 12/331,376,“Front Connected Photovoltaic Assembly and Associated Methods,” filedDec. 9, 2008; and Petti et al., U.S. patent application Ser. No.12/407,064, “Method to Make Electrical Contact to a Bonded Face of aPhotovoltaic Cell,” filed Mar. 19, 2009, hereinafter the '064application, both owned by the assignee of the present application andboth hereby incorporated by reference. If the methods of the '064application are employed, for example, gridlines 57 (formed by anysuitable method) make electrical contact to heavily doped p-typeamorphous silicon layer 74 by way of TCO 110, while contact is made tothe base of the cell by way of heavily doped n-type layer 14.

FIG. 3 f shows completed photovoltaic assembly 80, which includes aphotovoltaic cell and receiver element 60. In alternative embodiments,by changing the dopants used, heavily doped region 14 may serve as theemitter, at first surface 10, while heavily doped silicon layer 74serves as a contact to the base region. Incident light (indicated byarrows) falls on TCO 110, enters the cell at heavily doped p-typeamorphous silicon layer 74, enters lamina 40 at second surface 62, andtravels through lamina 40. In this embodiment, receiver element 60serves as a substrate. If receiver element 60 has, for example, a widestdimension about the same as that of lamina 40, the receiver element 60and lamina 40, and associated layers, form a photovoltaic assembly 80.Multiple photovoltaic assemblies 80 can be formed and affixed to asupporting substrate 90 or, alternatively, a supporting superstrate (notshown). Fabrication of this cell is described in more detail in Herner,U.S. patent application Ser. No. 12/540,463, “Intermetal Stack for Usein a Photovoltaic Device,” filed Aug. 13, 2009, owned by the assignee ofthe present application and hereby incorporated by reference.

This example was provided for completeness, but the structure and thefabrication process can be varied in many ways. For example, the stackbetween lamina 40 and receiver element 60 may be the intermetal stackdescribed by Herner et al. in U.S. patent application Ser. No.12/571,415, “Intermetal Stack For Use In a Photovoltaic Cell,” filedSep. 30, 2009, owned by the assignee of the present invention and herebyincorporated by reference. In this embodiment, shown in FIG. 4,photovoltaic assembly 81 also includes lamina 40 and receiver element60. The layers between lamina 40 and receiver element 60 may be, forexample, TCO layer 112; low-resistance layer 22, which may be nickel;non-reactive barrier layer 26, which may be titanium nitride; andadhesion layer 32, which may be titanium.

In another alternative embodiment, referring to FIG. 5, receiver element60 may be composed of a material like glass that is transparent tomicrowaves, and layers 52 between lamina 40 and receiver element 60 maybe of a material, such as a TCO or a dielectric such as silicon dioxide,that is transparent to microwaves. In these embodiments, some portion ofincident microwave energy 50 is absorbed by lamina 40.

The microwaves that traverse lamina 40 without being absorbed then passthrough the transparent layers 52 and the transparent receiver element60. As these layers are transparent to microwaves, the microwaves passthrough without causing direct heating. In such an embodiment, followingcompletion of fabrication, the structure may be inverted, and receiverelement 60 may serve as a superstrate in the completed device.

Microwave annealing may also be performed in many other embodiments,including those described in Sivaram et al., Herner, or Herner et al.,earlier incorporated, or, for example, Hilali et al., U.S. patentapplication Ser. No. 12/189,158, “Photovoltaic Cell Comprising a ThinLamina Having a Rear Junction and Method of Making,” filed Aug. 10,2008, owned by the assignee of the present application and herebyincorporated by reference. In some embodiments, receiver element 60 mayserve as a superstrate in the completed device.

In other embodiments, a plurality of donor wafers may be affixed to asingle receiver element, yielding multiple laminae, which are fabricatedinto photovoltaic cells as described. The photovoltaic cells may beelectrically connected in series, forming a photovoltaic module.

Summarizing, what has been described is a method to form a photovoltaiccell, the method comprising the steps of: defining a cleave plane in asubstantially crystalline semiconductor donor wafer; bonding the donorwafer at a first surface to a receiver element; cleaving a substantiallycrystalline semiconductor lamina from the donor wafer at the cleaveplane, wherein the lamina remains bonded to the receiver element at thefirst surface and a second surface of the lamina is created by cleaving,the second surface opposite the first, and wherein the lamina has athickness of at least 2 microns; and annealing the entire thickness ofthe bonded lamina with microwave energy to repair crystalline defects,wherein the lamina is suitable for use in the photovoltaic cell, thephotovoltaic cell comprising the lamina.

EXAMPLE Crystalize Amorphous Layer

In an alternative embodiment, a donor wafer is implanted with, forexample, hydrogen and/or helium ions to define a cleave plane, isaffixed to a receiver element with zero, one, or more layersintervening, and, referring to FIG. 6, a lamina 40 is cleaved from thedonor wafer at the cleave plane, remaining affixed to the receiverelement 60. As in prior embodiments, there may be zero, one, or morelayers between lamina 40 and receiver element 60, not shown here. Thedonor wafer, and thus lamina 40, is formed of substantially crystallinesemiconductor material, for example monocrystalline, multicrystalline,or polycrystalline silicon.

Following exfoliation and treatment of second surface 62 to removesurface damage, it is possible to add to the thickness of the lamina bydepositing a layer 54 of amorphous, nanocrystalline, or microcrystallinesemiconductor material such as silicon onto lamina 40. Layer 54 can be acombination of any of these, for example part nanocrystalline and partmicrocrystalline. Layer 54 can be annealed using microwave energy toincrease its degree of crystallinity. After annealing, layer 54 can be,for example, monocrystalline, multicrystalline, or polycrystalline. Thesame microwave anneal step will also cure crystal defects in lamina 40caused by the implant step. Layer 54 can be doped during deposition. Anamorphous layer can be deposited on top of layer 54 following amicrowave anneal, or need not be.

The final cell can be formed in a variety of configurations. The emitterregion of the cell, for example, which will be heavily doped to aconductivity type opposite the conductivity type of the base region ofthe cell, can be formed by doping at first surface 10 of lamina 40, nearreceiver element 60, before cleaving. Alternatively, layer 54 caninitially be lightly doped to a first conductivity type, then, duringthe final stages of deposition, the last thickness of layer 54 can beheavily doped to the opposite conductivity type, forming the emitter. Instill another alternative, Layer 54 can be lightly doped to a firstconductivity type and subjected to microwave anneal. Following microwaveanneal, an amorphous layer heavily doped to the opposite conductivitytype can be deposited on layer 54, forming the emitter. Otherconfigurations can be imagined.

Fabrication continues to complete a photovoltaic cell comprising lamina40 and receiver element 60, as described earlier.

EXAMPLE Activate Dopants

Other embodiments may include a shallow junction formed at the cleavedsurface, as described by Hilali et al., U.S. patent application Ser. No.12/399,065, “Photovoltaic Cell Comprising an MIS-Type Tunnel Diode,”filed Mar. 6, 2009, owned by the assignee of the present application andhereby incorporated by reference.

Such a shallow junction can be formed using a microwave anneal.Referring to FIG. 7, following formation of lamina 40 affixed toreceiver element 60 (interposed layers, if any, are not shown), andtreatment of second surface 62 to remove surface damage, a spin-on orspray-on dopant can be formed on second surface 62, including an n-typeor p-type dopant, or a dopant may be provided by shallow ionimplantation. A microwave anneal drives the dopant into second surface62 of lamina 40 and activates it, forming shallow doped region 16.Fabrication continues to form a photovoltaic cell, as described byHilali et al.

A variety of embodiments has been provided for clarity and completeness.Clearly it is impractical to list all possible embodiments. Otherembodiments of the invention will be apparent to one of ordinary skillin the art when informed by the present specification. Detailed methodsof fabrication have been described herein, but any other methods thatform the same structures can be used while the results fall within thescope of the invention.

The foregoing detailed description has described only a few of the manyforms that this invention can take. For this reason, this detaileddescription is intended by way of illustration, and not by way oflimitation. It is only the following claims, including all equivalents,which are intended to define the scope of this invention.

1. A method to form a photovoltaic cell, the method comprising the stepsof: providing a semiconductor lamina bonded to a receiver element, thelamina having a first surface and a second surface opposite the first,the thickness between the first surface and the second surface betweenabout 1.5 microns and about 10 microns, wherein the lamina is bonded tothe receiver element at the first surface, with zero, one, or morelayers intervening, and the second surface is exposed, and wherein thereceiver element has a thickness of at least 80 microns; and annealingthe entire thickness of the bonded lamina with microwave energy, whereinthe lamina is suitable for use in the photovoltaic cell, and wherein thephotovoltaic cell comprises the lamina.
 2. The method of claim 1 furthercomprising the step of fabricating the photovoltaic cell.
 3. The methodof claim 1 wherein the semiconductor lamina is substantiallycrystalline.
 4. The method of claim 1 wherein the receiver elementcomprises glass, metal, polymer, ceramic, or semiconductor material. 5.The method of claim 4 wherein the receiver element comprises glass. 6.The method of claim 1 wherein a metal layer is disposed between thelamina and the receiver element.
 7. The method of claim 6 wherein,during the annealing step, microwave energy enters the lamina at thesecond surface and is reflected from the metal layer.
 8. The method ofclaim 6 wherein a transparent conductive oxide layer is energy entersthe lamina at the second surface and is absorbed at a depth of at least2 disposed between the lamina and the metal layer.
 9. The method ofclaim 1 wherein, during the annealing step, microwave microns beneaththe second surface.
 10. The method of claim 1 wherein, during theannealing step, microwave energy enters the lamina at the second surfaceand is absorbed at a depth of at least 4 microns beneath the secondsurface.
 11. The method of claim 1 wherein the power of the microwaveenergy during the anneal step is between about 500 watts and about 5000watts.
 12. The method of claim 1 wherein, before the annealing step, thelamina is substantially monocrystalline silicon.
 13. The method of claim1 wherein, before the annealing step, the lamina includes an amorphoussilicon, nanocrystalline, or microcrystalline silicon layer.
 14. Amethod to form a photovoltaic cell, the method comprising the steps of:defining a cleave plane in a substantially crystalline semiconductordonor wafer; bonding the donor wafer at a first surface to a receiverelement; cleaving a substantially crystalline semiconductor lamina fromthe donor wafer at the cleave plane, wherein the lamina remains bondedto the receiver element at the first surface and a second surface of thelamina is created by cleaving, the second surface opposite the first,and wherein the lamina has a thickness of at least 2 microns; andannealing the entire thickness of the bonded lamina with microwaveenergy to repair crystalline defects, wherein the lamina is suitable foruse in the photovoltaic cell, the photovoltaic cell comprising thelamina.
 15. The method of claim 14 further comprising fabricating thephotovoltaic cell.
 16. The method of claim 14 wherein the receiverelement comprises glass, metal, polymer, ceramic, or semiconductormaterial.
 17. The method of claim 16 wherein the receiver elementcomprises glass.
 18. The method of claim 14 wherein a metal layer isdisposed between the lamina and the receiver element.
 19. The method ofclaim 18 wherein, during the annealing step, microwave energy enters thelamina at the second surface and is reflected from the metal layer. 20.The method of claim 18 wherein a transparent conductive oxide isdisposed between the lamina and the metal layer.
 21. The method of claim14 wherein the donor wafer is monocrystalline silicon.
 22. The method ofclaim 14 wherein, during the bonding step, zero, one, or more layers aredisposed between the donor wafer and the receiver element.
 23. Themethod of claim 14 further comprising, before the annealing step,etching at the second surface to remove exfoliation damage.